## Microbiology of Septic Arthritis **Key Point:** Staphylococcus aureus is the most common causative organism of acute septic arthritis in adults across all age groups and geographic regions, including India. ### Epidemiology - Accounts for 40–50% of all culture-positive septic arthritis cases - Responsible for both community-acquired and nosocomial infections - Increasingly includes methicillin-resistant S. aureus (MRSA) strains in hospital settings ### Other Common Organisms by Context | Organism | Clinical Context | Frequency | |----------|------------------|----------| | S. aureus | All ages, all settings | 40–50% | | Streptococcus spp. | Post-surgical, immunocompromised | 10–15% | | Gram-negative bacilli | Elderly, immunocompromised, IV drug users | 10–20% | | N. gonorrhoeae | Young sexually active patients | 5–10% | | M. tuberculosis | Chronic monoarthritis (TB-endemic regions) | <5% (chronic) | **High-Yield:** While M. tuberculosis causes septic arthritis in India, it presents as a **chronic monoarthritis** (weeks to months), not acute septic arthritis. Acute septic arthritis with rapid onset and systemic toxicity is dominated by S. aureus. ### Clinical Pearl **Warning:** Do not confuse acute bacterial septic arthritis with tuberculous arthritis. TB arthritis is insidious, often monoarticular (knee, hip, spine), and has a subacute–chronic course with constitutional symptoms developing over weeks. Acute septic arthritis presents with fever, joint pain, and swelling within days. **Mnemonic:** **GNSS** — Gram-positive cocci (S. aureus, Streptococcus), Gram-negative bacilli, and Neisseria are the "big four" in acute septic arthritis; TB is separate (chronic). 
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